During construction of oil and gas wells, a rotary drill is typically used to bore through subterranean formations of the earth to form a borehole. As the rotary drill bores through the earth, a drilling fluid or mud is circulated through the borehole. Drilling fluids are usually pumped from the surface through the interior of the drill pipe. By continuously pumping the drilling fluid through the drill pipe, the drilling fluid can be circulated out the bottom of the drill pipe and back up to the well surface through the annular space between the wall of the well bore and the drill pipe.
Once the wellbore has been drilled, casing is lowered into the wellbore. A cement slurry is then pumped into the casing and a plug of fluid, such as drilling mud or water, is then pumped behind the cement slurry in order to force the cement up into the annulus between the exterior of the casing and the borehole. The cement slurry is then allowed to harden as a sheath. The cement sheath then holds the casing in place. The well is subsequently stimulated in order to enhance the recovery of oil or gas from the reservoir.
During well treatment operations, including stimulation operations, cement sheaths are subjected to axial, shear and compressional stresses induced by vibrations and impacts. In particular, stress conditions may be induced or aggravated by fluctuations or cycling in temperature or fluid pressures. In addition, variations in temperature and internal pressure of the wellbore pipe string may result in radial and longitudinal pipe expansion and/or contraction. This tends to place stress on the annular cement sheath existing between the outside surface of the pipe string and the inside formation surface or wall of the wellbore. Such stresses lead to cracking and/or disintegration of the cement sheath.
Not only must the cement slurry have a pumpable viscosity, acceptable fluid loss control, minimal settling of particles and the ability to set within a practical time, the cement mix and the properties of the cement slurry must be carefully selected in order to minimize or eliminate cracking of the cement sheath. As such, the cement mix and the slurry containing the mix must be tailored in order for the cement sheath to withstand those axial stresses, shear stresses and compressional stresses encountered under in-situ wellbore conditions. Further, the components of the cement mix and the cement slurry must be selected such that, when hardened, the cement sheath is not brittle since brittleness causes cracking of the sheath.
Thus, it has become increasingly important for service providers to provide to well operators cement mixes capable of withstanding specific downhole conditions well and specific operating conditions which the well is to be subjected.
Several testing methods have been developed to date to test physical properties of cured cements. For example, ASTM International has established the Standard Test Method for Flexural Strength of Concrete (Using Simple Beam With Center-Point Loading), Designation No. C 293-02. This test method purports to accurately determine the flexural strength of a set cement specimen by the use of a simple beam with center-point loading. The method employs a load-applying block and two specimen support blocks wherein force is applied perpendicular to the face of the specimen until the specimen fails. The modulus of rupture is calculated as:R=3PL/2bd2  (1)wherein:
R=Modulus of rupture, psi, or MPa,
P=maximum applied load indicated by the testing machine, lbf, or N,
L=span length, in., or mm,
b=average width of the specimen at the fracture, in., or mm; and
d=average depth of the specimen a the fracture, in., or mm.
However, the method only provides a modulus of rupture based on a perpendicular force being applied in surface ambient conditions. The method therefore fails to simulate the stresses encountered in the higher temperature and pressure conditions of the wellbore environment.
Additional standards have been developed for testing cement. For example ASTM International Standard Test Method for Flexural Strength of Hydraulic-Cement Mortars, Designation No. C 348-02 provides a centerpoint loading such that forces are applied to the specimen in a vertical direction to determine the flexural strength from the total maximum load as follows:Sf=0.0028P  (2)wherein:
Sf=flexural strength, Mpa, and
P=total maximum load, N.
This method only provides a flexural strength based on a vertical force being applied in surface ambient conditions to cause a total maximum load. This method therefore also fails to simulate the stresses encountered in the higher temperature and pressure conditions of the wellbore environment.
Mechanical properties of set cement have further been predicted using a curing chamber which purports to simulate downhole wellbore conditions. In most cases, after hardening of the slurry the test temperature and pressure are slowly decreased to accommodate the safe removal of test sample from the curing chamber. In light of such changing conditions, the data is less than accurate.
A testing protocol is desired which does not introduce experimental errors into the procedure. In particular, a testing protocol is desired for assessing mechanical properties of a cement sheath at simulated conditions and at conditions found in the wellbore environment.
A need exists for a testing method for hardened cements under conditions which simulate conditions found in a wellbore environment. Testing methods under these conditions will provide the requisite data for optimizing the properties of cementitious slurries for rendering suitable hardened cements at in-situ stress conditions.